Optical transmitter for increased effective modal bandwidth transmission

CROSS REFERENCE TO RELATED APPLICATIONS

0001This patent application claims priority to U.S. provisional patent application Ser. No. 60/481,166, filed on Aug. 1, 2003, and entitled “Optical Fiber Transmission System with Increased Effective Modal Bandwidth,” the entire disclosure of which is incorporated herein by reference.

BACKGROUND OF INVENTION

0002Many existing optical fiber transmission systems use multi-mode optical fiber. Multi-mode optical fiber is widely used because it is relatively inexpensive, easy to install and because it is suitable for use with low cost transmitter and receiver components. The relatively large optical fiber core and numerical aperture of multi-mode optical fibers allows more light to be launched into the optical fiber, as compared to single-mode optical fibers. Therefore, such systems can use lower power and lower cost optical sources. For these reasons, local area networks have employed multi-mode optical fiber for many years. Some data communication systems, such as Fiber Data Distribution Interface (FDDI) systems are specifically designed to use multi-mode optical fiber. Known multi-mode optical fiber transmission systems, however, have relatively low bandwidth-distance products for a given bit error rate (BER) and, therefore, are not suitable for many state-of-the art communication systems.

BRIEF DESCRIPTION OF DRAWINGS

This invention is described with particularity in the detailed description. The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in various figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

<figref idref="DRAWINGS">FIG. 1</figref> illustrates a block diagram of a multi-mode optical fiber transmission system that includes two spatial mode optical filters according to the present invention.

<figref idref="DRAWINGS">FIG. 2</figref> illustrates a block diagram of a single-mode optical fiber transmission system that includes a spatial mode optical filter according to the present invention.

<figref idref="DRAWINGS">FIG. 3</figref> illustrates a block diagram of an optical transmitter that includes an electro-absorption modulator according to the present invention that generates optical signals with improved or optimal spectral and phase characteristics for transmission through an optical fiber link.

<figref idref="DRAWINGS">FIG. 4</figref> illustrates a block diagram of an optical transmitter that includes an electro-absorption modulated laser (EML) according to the present invention that generates optical signals with improved or optimal spectral and phase characteristics for transmission through an optical fiber link.

<figref idref="DRAWINGS">FIG. 5</figref> illustrates a block diagram of an optical transmitter that includes an embodiment of a laser modulator according to the present invention that generates optical signals with improved or optimal spectral and phase characteristics for transmission through an optical fiber link.

DETAILED DESCRIPTION

0009The present invention relates to methods and apparatus for increasing the effective modal bandwidth of optical fiber transmission systems. The term “effective modal bandwidth” is defined herein to mean the bandwidth-distance product of the transmission system for a given Bit Error Rate (BER) and/or a certain transmission specification. Increasing the effective modal bandwidth of a multi-mode optical fiber transmission system will allow providers to increase the data rate and will extend the useful service life of many installed multi-mode optical fiber transmission systems.

0010One aspect of the present invention is embodied in the design of optical transmitters that have improved or optimum mum spectral and phase characteristics for transmitting data in a multi-mode optical fiber. Another aspect of the present invention is embodied in the use of spatial filtering to reduce the number of modes propagating in a multi-mode optical fiber. These aspects alone or in combination increase the effective modal bandwidth of multi-mode optical fiber transmission systems.

0011<figref idref="DRAWINGS">FIG. 1</figref> illustrates a block diagram of a multi-mode optical fiber transmission system <b>100</b> that includes two spatial mode optical filters according to the present invention. The transmission system <b>100</b> includes an optical transmitter <b>102</b>, a multi-mode optical fiber link <b>104</b>, and an optical receiver <b>106</b>. The optical transmitter <b>102</b> generates optical signals for data transmission through the multi-mode optical fiber link <b>104</b>.

0012In one embodiment, the optical transmitter <b>102</b> includes an intensity modulated optical source, an electro-absorption modulated laser, an integrated laser modulator, or a laser modulator having parameters that generate optical signals with improved or optimal spectral and phase characteristics for transmission through an optical fiber link as described herein. In one embodiment, the optical transmitter <b>102</b> includes more than one optical source that generates additional optical signals at different wavelengths. In one embodiment, the optical source includes a WDM optical source that generates a plurality of optical signals and each of the plurality of optical signals has a different wavelength.

0013In some embodiments, the optical transmitter <b>102</b> includes additional optical sources that are used to generate additional optical signals that increase the data capacity of the multi-mode optical fiber link <b>104</b>. In some of these embodiments, the multi-mode optical fiber transmission system <b>100</b> includes additional optical transmitters <b>102</b> that are used to generate optical signals that propagate in opposite directions in the same multi-mode optical fiber. Separate optical carriers can be used to minimize cross-talk between optical signals propagating in opposite directions.

0014The optical transmitter <b>102</b> is optically coupled to a first single-mode optical fiber <b>108</b>. Optical signals generated by the optical transmitter <b>102</b> propagate down the first single-mode optical fiber <b>108</b>. A first spatial mode converter <b>110</b> is optically coupled to the first single-mode optical fiber <b>108</b>. The first spatial mode converter <b>110</b> reduces the number of modes in the optical signal propagating through the first spatial mode converter <b>110</b>.

0015The first spatial mode converter <b>110</b> can reduce the number of higher-order modes, the number of lower-order modes or both the number of higher-and lower-order modes in the optical signal propagating through the first spatial mode converter <b>110</b>. By lower-order modes, we mean modes in which most of the energy is localized around the center of the optical fiber core of the multi-mode optical fiber. By higher-order modes, we mean modes in which most of the energy is localized outside of the center of the optical fiber core of the multi-mode optical fiber.

0016An input <b>112</b> of the multi-mode optical fiber link <b>104</b> is optically coupled to the first spatial mode converter <b>110</b>. The multi-mode optical fiber link <b>104</b> can include a single length of multi-mode optical fiber or can include multiple lengths of multi-mode optical fiber that are coupled together. The multiple lengths of multi-mode optical fiber can be butt coupled together. For example, the butt couplings can be tapered optical fiber sections or polished optical fiber sections.

0017A second spatial mode converter <b>116</b> is optically coupled to an output <b>114</b> of the multi-mode optical fiber link <b>104</b>. The second spatial mode converter <b>116</b> is also optically coupled to a second single-mode optical fiber <b>118</b>. The second spatial mode converter <b>116</b> further reduces the number of modes in the optical signal that are transmitted through the second spatial mode converter <b>116</b> and, therefore, limits the number of dominant modes that will be received by the optical receiver <b>106</b>. The second spatial mode converter <b>116</b> can reduce the number of higher-order modes, the number of lower-order modes or both the number of higher-and lower-order modes in the optical signal propagating through the second spatial mode converter <b>116</b>.

0018Both the first <b>110</b> and the second spatial mode converters <b>116</b> increase the effective modal bandwidth of the multi-mode optical fiber transmission system <b>100</b>. The first and second spatial mode converters <b>110</b>, <b>116</b> can be any type of spatial mode converter that reduces the number of modes in the optical signal generated by the optical transmitter <b>102</b>. For example, the first and second spatial mode converters <b>110</b>, <b>116</b> can include a fusion splice or a butt coupling between the multi-mode optical fiber <b>104</b> and a respective one of the first <b>108</b> and the second single-mode optical fiber <b>118</b>. The butt coupling can be positioned at a bulkhead. The first and second spatial mode converters <b>110</b>, <b>116</b> can also include a lens imaging system having refractive and diffractive elements.

0019The effective modal bandwidth of the multi-mode optical fiber transmission system <b>100</b> according to the present invention including the two spatial mode converters <b>110</b>, <b>116</b> has a relatively high-level of immunity to polarization effects, fiber stress, vibration, and changes in temperature. In particular, there is little or no change in the effective modal bandwidth due to changes in laser polarization or changes in polarization caused by mechanical stress on the multi-mode optical fiber link <b>104</b>. Also, there is little or no change in the effective modal bandwidth due to temperature changes in the fiber environment.

0020The present invention features a method of increasing effective modal bandwidth of an optical signal transmitted through a multi-mode optical fiber. The method includes generating an optical signal and propagating the optical signal through a single-mode optical fiber. In one embodiment, the optical signal is chosen to reduce phase corruption. The optical signal is then spatially mode converted to an optical signal having a lower number of modes. The spatial mode converting reduces modal dispersion, which increases an effective bandwidth of the optical signal.

0021The optical signal having the lower number of modes is then propagated through a multi-mode optical fiber. The optical signal propagating through the multi-mode optical fiber is then spatially mode converted, which further increases the effective bandwidth of the optical signal. The spatial mode conversions can reduce changes in effective modal bandwidth of the optical signal that are caused by physical effects, such as thermal variations in the multi-mode optical fiber, polarization effects in the multi-mode optical fiber, mechanical stress in the multi-mode optical fiber, optical fiber splices in the multi-mode optical fiber, and optical connector misalignment in the multi-mode optical fiber.

0022Some aspects of the present invention are described in connection with a multi-mode optical fiber link that is typically a local area fiber link. However, the present invention can also be practiced with a single-mode optical fiber link that is typically a long-haul optical fiber link.

0023<figref idref="DRAWINGS">FIG. 2</figref> illustrates a block diagram of a single-mode optical fiber transmission system <b>150</b> that includes a spatial mode optical filter according to the present invention. The transmission system <b>150</b> includes an optical transmitter <b>102</b>, a single-mode optical fiber link <b>152</b>, and an optical receiver <b>106</b>. The single-mode optical fiber transmission system <b>150</b> is similar to the multi-mode optical fiber transmission system <b>150</b> that was described in connection with <figref idref="DRAWINGS">FIG. 1</figref>.

0024The optical transmitter <b>102</b> generates optical signals for data transmission through the single-mode optical fiber link <b>152</b>. In one embodiment, the optical transmitter <b>102</b> includes more than one optical source that generates additional optical signals at different wavelengths that increase the data capacity of the single-mode optical fiber link <b>152</b>. In some embodiments, the single-mode optical fiber transmission system <b>100</b> includes additional optical transmitters <b>102</b> that are used to generate optical signals that propagate in opposite directions in the same single-mode optical fiber.

0025The optical transmitter <b>102</b> is optically coupled to a first single-mode optical fiber <b>108</b>. Optical signals generated by the optical transmitter <b>102</b> propagate down the first single-mode optical fiber <b>108</b>. A first spatial mode converter <b>110</b> is optically coupled to the first single-mode optical fiber <b>108</b>. The first spatial mode converter <b>110</b> reduces the number of modes in the optical signal propagating through the first spatial mode converter <b>110</b>.

0026An input <b>151</b> of the single-mode optical fiber link <b>152</b> is optically coupled to the first spatial mode converter <b>110</b>. The single-mode optical fiber link <b>152</b> can include a single length of single-mode optical fiber or can include multiple lengths of single-mode optical fiber that are fusion spliced or coupled together. An optical coupler <b>154</b> is optically coupled to an output <b>156</b> of the single-mode optical fiber link <b>152</b>. The optical coupler <b>154</b> is also optically coupled to a second single-mode optical fiber <b>118</b>.

0027The first spatial mode converter <b>110</b> increases the effective modal bandwidth of the single-mode optical fiber transmission system <b>150</b>. The effective modal bandwidth of the single-mode optical fiber transmission system <b>150</b> according to the present invention including the first spatial mode converter <b>110</b> has a relatively high-level of immunity to polarization effects, fiber stress, and changes in temperature.

0028Dispersion can degrade signals in the multi-mode and single-mode optical fiber transmission systems <b>100</b>, <b>150</b> that are described in connection with <figref idref="DRAWINGS">FIG. 1</figref> and <figref idref="DRAWINGS">FIG. 2</figref>. Several different types of dispersion can occur in these optical fiber transmission systems. For example, chromatic dispersion can occur in WDM optical fiber transmission systems. Chromatic dispersion is caused by differences in the speed at which signals having different wavelengths travel in the optical fiber link. Chromatic dispersion generally decreases the acceptable transmission distance as the square of the bit rate.

0029Polarization mode dispersion (PMD) occurs when the orthogonal polarization components of the optical signal travel at different rates in the optical fiber link. Polarization mode dispersion results from asymmetries in the optical fiber core. Polarization mode dispersion causes a statistical disruption in network operation and, consequently, limits the transmission distance.

0030Signal degradation caused by these dispersions, if uncompensated, corrupts the signal by broadening the pulses in the signal, which causes Inter Symbol Interference (ISI). The ISI will eventually degrade the signal quality enough for the signal to fall below the acceptable threshold for service. Thus, these dispersions can limit the possible transmission distance in the optical fiber links and can cause service interruptions.

0031The multi-mode and single-mode optical fiber transmission systems that are described in connection with <figref idref="DRAWINGS">FIG. 1</figref> and <figref idref="DRAWINGS">FIG. 2</figref> can be used in conjunction with any type of dispersion compensation technique. In one embodiment of the present invention, electronic dispersion compensation is used to reconstruct dispersed signals received by the receiver <b>106</b>. In this embodiment, the receiver <b>106</b> includes at least one active filter that is electrically coupled to the output of a detector. There are many different types of active filters know in the art that are suitable for electronic dispersion compensation.

0032For example, the active filter can be a Finite Impulse Response (FIR) filter, such as a Feed Forward Equalizer (FFE) filter. Such filters sample the received signal, after electro-optic conversion by the detector. Different delayed samples are scaled and then summed once per sample clock. The length of the FIR filter (i.e. the number of taps) is related to the amount of ISI that is incurred during transmission.

0033A Decision Feedback Equalizer (DFE) filter can be used with the FFE filter to further reduce the ISI. The DFE filter takes the decisions from the FFE filter as its input. The output of the DFE is combined with the output of the FFE filter and is fed back to the input of the DFE. The clock and data are then recovered from the dispersion compensated signal. The received data is then demultiplexed.

0034<figref idref="DRAWINGS">FIG. 3</figref> illustrates a block diagram of an optical transmitter <b>200</b> that includes an electro-absorption modulator according to the present invention that generates optical signals with improved or optimal spectral and phase characteristics for transmission through an optical fiber link. The optical transmitter <b>200</b> improves the spectral and phase characteristics for transmission through multi-mode optical fiber links, such as the multi-mode optical fiber link that is described in connection with <figref idref="DRAWINGS">FIG. 1</figref>. In addition, the optical transmitter <b>200</b> improves the spectral and phase characteristics for transmission through single-mode optical fiber links, such as long-haul single-mode optical fiber links.

0035The optical transmitter <b>200</b> is designed to generate optical signals that have specific characteristics which increase or maximize immunity to variations in the phase of the optical signal received by the optical receiver <b>106</b> (<figref idref="DRAWINGS">FIGS. 1 and 2</figref>). One characteristic of the optical signal generated by the optical transmitter <b>200</b> is a reduction in time varying phase or sideband information in the transmission spectrum of the optical signal. Another characteristic of the optical signal generated by the optical transmitter <b>200</b> is a reduction in the phase information that is required to transmit the data in the optical fiber links <b>104</b>, <b>152</b> (<figref idref="DRAWINGS">FIGS. 1 and 2</figref>).

0036Another characteristic of the optical signal generated by the optical transmitter <b>200</b> is a reduction or elimination of mixing that is required at the optical receiver <b>106</b> (<figref idref="DRAWINGS">FIG. 1</figref>) to recover the optical signal. Yet another characteristic of the optical signal generated by the optical transmitter <b>200</b> is an increase in isolation of optical signals reflected back towards the optical transmitter <b>102</b>. In one embodiment of the invention, the optical transmitter <b>200</b> generates an optical signal with one or any combination of these characteristics. Generating an optical signal with one or more of these characteristics will increase the effective modal bandwidth of the multi-mode optical fiber transmission system <b>100</b> (<figref idref="DRAWINGS">FIG. 1</figref>) and the effective modal bandwidth of the single-mode optical fiber transmission system (<figref idref="DRAWINGS">FIG. 2</figref>).

0037One type of optical transmitter that can generate an optical signal with one or any combination of these characteristics is an electro-absorptively (EA) modulated optical transmitter. The optical transmitter <b>200</b> illustrated in <figref idref="DRAWINGS">FIG. 3</figref> is an exemplary EA optical modulated transmitter. Numerous types of EA modulated sources can be used in an optical transmitter according to the present invention. In other embodiments, other types of intensity modulators are used.

0038The optical transmitter <b>200</b> includes a laser <b>202</b> that generates a continuous wave (CW) optical signal at an output <b>204</b>. In some embodiments, the laser <b>202</b> is a semiconductor diode laser. However, other types of lasers can also be used. The transmitter <b>200</b> also includes a laser bias circuit <b>206</b>. An output <b>208</b> of the laser bias circuit <b>206</b> is electrically connected to a bias input <b>210</b> of the laser <b>202</b>. The laser bias circuit <b>206</b> generates a current at the output <b>208</b> that biases the laser <b>202</b>.

0039The optical transmitter <b>200</b> also includes an Electro-Absorption Modulator (EAM) <b>210</b> that modulates the CW optical signal generated by the laser <b>202</b>. In some embodiments, the laser <b>202</b> and the EAM <b>210</b> are separate discrete components. In other embodiments, the laser <b>202</b> and the EAM <b>210</b> are physically integrated on a single substrate. The EAM <b>210</b> includes an optical input <b>212</b>, a bias and modulation input <b>214</b>, and an optical output <b>216</b>. The optical input <b>212</b> is positioned in optical communication with the output <b>204</b> of the laser <b>202</b>. A waveguide, such as an optical fiber, can be used to optically couple the output <b>204</b> of the laser <b>202</b> to the optical input <b>212</b> of the EAM <b>210</b>.

0040The optical transmitter <b>200</b> including the EAM <b>210</b> generates optical signals with improved or optimal spectral and phase characteristics for transmission through a multi-mode optical fiber link. The modulated optical signal that is generated by the optical transmitter <b>200</b> including the EAM <b>210</b> has very little phase information because EA modulators operate as efficient intensity modulators.

0041In one embodiment of the invention, the EAM <b>210</b> is specifically designed and fabricated to have at least one parameter that causes the EAM <b>210</b> to modulate intensity so as to suppress phase and sideband information in the transmission spectrum. EA modulators are relatively efficient intensity modulators. Therefore, time varying phase and sideband information in the transmission spectrum is generally suppressed. However, a transmitter according to one embodiment of the invention can be designed, fabricated, and/or operated to further reduce phase and sideband information in the transmission spectrum.

0042There are numerous physical EA modulator parameters that can be adjusted to change the amplitude and phase characteristics of the modulated optical signal in order to suppress phase and sideband information from the transmission spectrum. For example, parameters, such as the extinction ratio or voltage swing of the EA modulator, polarization properties, the 3-dB bandwidth, the facet coating properties, the input third-order intercept (IIP3), and the spurious free dynamic range (SFDR) can be adjusted during design and fabrication to suppress phase and sideband information from the transmission spectrum. Adjusting the extinction ratio of the EA modulator has been shown to suppress phase and sideband information from the transmission spectrum and, consequently, to increase the signal-to-noise ratio of optical signals propagating through multi-mode optical fiber. In one embodiment of the invention, the extinction ratio of the EA modulator <b>210</b> is in the range of about five to fifteen.

0043The optimal value of the extinction ratio is a function of the length of the multi-mode optical fiber. The optimal value of the extinction ratio can also be a function of the number of the fiber connectors and the alignment of the fiber connectors in the multi-mode optical fiber link <b>104</b> (<figref idref="DRAWINGS">FIG. 1</figref>) and the single mode optical fiber link <b>152</b> (<figref idref="DRAWINGS">FIG. 2</figref>). In addition, the optimal value of the extinction ratio can also be a function and many environmental factors, such as the level of the vibration, mechanical strain, thermal shock, and optical power fluctuations in the optical fiber link.

0044In one example, an EA modulator with an extinction ratio of about 11.5 has been shown to transmit optical signals through a 1250 foot multi-mode optical fiber link with relatively low phase and sideband information and relatively high signal-to-noise ratio compared with EA modulators having extinction ratios of about five and about eight in the same optical fiber link under similar environmental conditions. In another example, an EA modulator with an extinction ratio of about ten has been shown to transmit optical signals through a 4500 foot multi-mode optical fiber link with relatively low phase and sideband information and relatively high signal-to-noise ratio compared with an EA modulator having an extinction ratio of about five in the same optical fiber link under similar environmental conditions.

0045The optical transmitter <b>200</b> also includes a bias and data multiplexing circuit <b>218</b> that generates the desired electrical bias and data signals for the EAM <b>210</b>. In some embodiments, the bias and data multiplexing circuit <b>218</b> includes two physically separate components. In other embodiments, the bias and data multiplexing circuit <b>218</b> is one component as shown in <figref idref="DRAWINGS">FIG. 3</figref>. An output <b>220</b> of the bias and data multiplexing circuit <b>218</b> is electrically connected to the modulation input <b>214</b> of the EAM <b>210</b>. The EAM <b>210</b> modulates the CW optical signal generated by the laser <b>202</b> with an electronic data signal generated by the bias and data multiplexing circuit <b>218</b>. The modulated optical signal propagates from the optical output <b>216</b> of the EAM <b>210</b>.

0046In one embodiment of the invention, the operating conditions of the EAM <b>210</b> are chosen so as to suppress phase and/or sideband information in the transmission spectrum generated by the EAM <b>210</b>. For example, the operating temperature of the EAM <b>210</b> and the bias voltage that is generated by the bias and data multiplexing circuit <b>218</b> and applied to the modulation input <b>214</b> of the EAM <b>210</b> can be adjusted during operation to suppress phase and/or sideband information from the optical signal.

0047In addition, parameters of the laser <b>202</b> that generates the optical signal which is modulated by the EAM <b>210</b> can be adjusted to suppress phase and/or sideband information from the transmission spectrum. For example, parameters, such as the wavelength and the optical mode structure of the optical signal generated by the laser <b>202</b> can be adjusted so as to suppress phase information and/or sideband information from the modulated optical signal.

0048The modulated optical signal that is generated by the optical transmitter <b>200</b> including the EAM <b>210</b> has certain characteristics in its transmission spectrum that increase the effective modal bandwidth of the optical fiber link. For example, one characteristic of the transmission spectrum is that the optical signal has minimal time varying phase. Another characteristic of the transmission spectrum is that it has minimal sideband information.

0049The modulated optical signal that is generated by the optical transmitter <b>200</b> requires essentially no phase information to transmit the data in an optical link, such as the multi-mode optical fiber link <b>104</b> (<figref idref="DRAWINGS">FIG. 1</figref>) and the single-mode optical fiber link <b>152</b> (<figref idref="DRAWINGS">FIG. 2</figref>). In addition, the modulated optical signal that is generated by the optical transmitter <b>200</b> has good isolation from optical signals reflecting back towards the optical transmitter <b>200</b>.

0050Thus, the optical transmitter <b>200</b> improves the spectral and phase characteristics for transmission through multi-mode optical fiber links, such as the multi-mode optical fiber link <b>104</b> that is described in connection with <figref idref="DRAWINGS">FIG. 1</figref>. In addition, the optical transmitter <b>200</b> improves the spectral and phase characteristics for transmission through single-mode optical fiber links, such as the single-mode optical fiber link <b>152</b> that is described in connection with <figref idref="DRAWINGS">FIG. 2</figref>.

0051There are numerous other types of optical transmitters that when designed, fabricated, and operated according to the present invention will generate optical signals with improved or optimal spectral and phase characteristics for transmission through a multi-mode and single-mode optical fiber link. These optical transmitters include electro-absorption modulated lasers (EMLs), laser modulators, and electro-optic modulators.

0052<figref idref="DRAWINGS">FIG. 4</figref> illustrates a block diagram of an optical transmitter <b>250</b> that includes an electro-absorption modulated laser (EML) <b>252</b> according to the present invention that generates optical signals with improved or optimal spectral and phase characteristics for transmission through an optical fiber link. The EML <b>252</b> includes a laser diode <b>254</b> section and an electro-absorption modulator (EAM) <b>256</b> section.

0053The laser diode <b>254</b> section is typically a distributed feedback (DFB) laser. The EAM <b>256</b> is typically a device that includes a semiconductor layer, such as a multi-quantum well semiconductor layer. The semiconductor layer typically has a slightly larger absorption band edge than the photon energy of the light being modulated. The laser diode <b>254</b> section is optically coupled to the EAM <b>256</b> section. The laser diode <b>254</b> section and EAM section <b>256</b> are typically integrated onto a single substrate, but can be physically separate devices.

0054A laser bias circuit <b>258</b> has an output <b>260</b> that is electrically coupled to a bias input <b>262</b> of the laser diode <b>254</b>. The laser bias circuit <b>258</b> generates a continuous wave (CW) current that drives the laser diode <b>254</b>, thereby causing the laser diode <b>254</b> to emit substantially monochromatic light of a predetermined wavelength.

0055A modulator bias and data multiplexing circuit <b>264</b> has an output <b>266</b> that is electrically coupled to a modulation input <b>268</b> of the EAM <b>256</b>. The modulator bias and data multiplexing circuit <b>264</b> generates a voltage across the multi-quantum well semiconductor layer that produces a reverse bias modulating electric field across the semiconductor layer of the EAM <b>256</b>. The reverse bias modulating electric field causes the absorption edge of the semiconductor layer of the EAM <b>256</b> to reversibly move to a longer wavelength, which corresponds to a lower absorption edge. The lower absorption edge causes the semiconductor layer of the EAM <b>256</b> to absorb the light generated by the laser diode <b>254</b> section that propagates through the semiconductor layer of the EAM <b>256</b>.

0056Reducing the voltage across the multi-quantum well semiconductor layer results in the elimination or reduction of the reverse bias electric field, which causes the semiconductor layer of the EAM <b>256</b> to allow light generated by the laser diode <b>254</b> to transmit through the semiconductor layer of the EAM <b>256</b>. Therefore, light emitted from the laser diode <b>254</b> that propagates to the EAM <b>256</b> is modulated by modulating the voltage across the multi-quantum well semiconductor layer of the EAM <b>256</b>. The light emitted is modulated between a sufficient reverse bias voltage across the semiconductor layer that causes the layer to be substantially opaque to the light emitted from the laser diode <b>254</b>, and substantially zero or a sufficiently positive bias voltage that causes the layer to be substantially transparent to the light emitted from the laser diode <b>254</b>.

0057The resulting modulated light is emitted at an optical output <b>270</b> of the EML <b>252</b>. The optical output <b>270</b> is directly coupled to the single-mode optical fiber <b>108</b> (<figref idref="DRAWINGS">FIGS. 1 and 2</figref>). The wavelength of the modulated light can be controlled by adjusting the amplitude of the CW current generated by the laser bias circuit <b>258</b> and applied to the laser diode <b>254</b>. The wavelength of the modulated light can also be controlled by adjusting the temperature of the laser diode <b>254</b>.

0058The EML <b>252</b> includes a thermoelectric cooler (TEC) <b>272</b> that controls the temperature of the laser diode <b>254</b> and the EAM <b>256</b>. The temperature of the EML <b>252</b> can be stabilized by using a thermal sensor <b>274</b> and a feedback circuit <b>276</b>. The thermal sensor <b>274</b> is thermally coupled to the laser diode <b>254</b> and is electrically coupled to the feedback circuit <b>276</b>. The feedback circuit <b>276</b> is electrically coupled to the TEC <b>272</b>. The feedback circuit <b>276</b> receives a signal from the thermal sensor <b>274</b> that is related to the temperature of the laser diode <b>254</b> and generates a signal in response to the temperature. The signal generated by the feedback circuit <b>276</b> controls the thermal properties of the TEC <b>272</b> to maintain the laser diode <b>254</b> at a predetermined operating temperature (and thus the major portion of spectral energy of the emitted light at the desired wavelength) independent of ambient temperature.

0059In one embodiment of the invention, the EML <b>252</b> is specifically designed and fabricated to have at least one parameter that causes the EML <b>252</b> to generate a transmission spectrum with suppressed phase and sideband information. There are numerous physical EML parameters that can be adjusted to change the amplitude and phase characteristics of the modulated optical signal in order to suppress phase and sideband information from the transmission spectrum.

0060For example, parameters of the EAM <b>256</b>, such as the extinction ratio, the polarization properties, the 3-dB bandwidth, the modulator chirp, the optical mode structure, the input third-order intercept (IIP3), the spurious free dynamic range (SFDR), and the output facet coating properties can be adjusted during design and fabrication to suppress phase and/or sideband information from the transmission spectrum.

0061Also, parameters of the laser diode <b>254</b>, such as the wavelength, the optical mode structure, and the parameters of the output facet coating can be adjusted during design and fabrication to suppress phase and/or sideband information from the transmission spectrum. In addition, parameters specific to EML devices, such as the electrical isolation and the optical coupling between the laser diode <b>254</b> and the EAM <b>256</b> can be adjusted during design and fabrication to suppress phase and/or sideband information from the transmission spectrum.

0062In one embodiment of the invention, the operating conditions of the EML <b>252</b> are chosen so as to suppress phase and/or sideband information in the transmission spectrum. For example, operating conditions, such as the current generated by the laser bias circuit <b>258</b> and the resulting optical power received by the EAM <b>256</b>, the bias voltage swing that is generated by the bias and data multiplexing circuit <b>264</b> and received by the EAM <b>256</b>, and the operating temperature of the laser diode <b>254</b> and the EAM <b>256</b> can be adjusted during operation of the EML <b>252</b> to suppress phase and/or sideband information in the transmission spectrum.

0063The present invention can also be practiced with numerous types of laser modulators. <figref idref="DRAWINGS">FIG. 5</figref> illustrates a block diagram of an optical transmitter <b>300</b> that includes an embodiment of a laser modulator <b>302</b> according to the present invention that generates optical signals with improved or optimal spectral and phase characteristics for transmission through an optical fiber link. The optical transmitter <b>300</b> includes a laser section <b>304</b> and a modulator section <b>306</b>.

0064The laser section <b>304</b> of the laser modulator <b>302</b> shown in <figref idref="DRAWINGS">FIG. 5</figref> is a tunable three section Distributed Bragg Reflector (DBR) laser. In other embodiments (not shown) a single section DFB laser can be used if wavelength tuning is not desirable. The laser section <b>304</b> includes a gain section <b>308</b>, a phase section <b>310</b>, and a grating section <b>312</b> that are butted together. The gain section <b>308</b> generates an optical signal. The phase section <b>310</b> introduces an optical phase shift to tune the laser wavelength. The grating section <b>312</b> forms a DBR mirror.

0065A high reflection coating <b>314</b> is deposited on one side of the gain section <b>308</b>. A laser cavity is formed between the high reflection coating <b>314</b> and the DBR mirror formed by the grating section <b>312</b>. The optical transmitter <b>300</b> includes a laser bias circuit <b>316</b> having an output <b>318</b> that is electrically connected to a bias input <b>320</b> of the gain section <b>308</b>. The laser bias circuit <b>316</b> generates a current at the output <b>318</b> that biases the gain section <b>308</b> to emit the desired optical signal. The design and operation of such lasers are well known in the art.

0066The modulator section <b>306</b> of the laser modulator <b>302</b> is positioned outside of the laser cavity beyond the DBR mirror in the grating section <b>312</b>. Forming the modulator section <b>306</b> external to the laser cavity introduces relatively low wavelength chirp into the modulated optical signal. An input <b>328</b> of the modulator section <b>306</b> is optically coupled to the grating section <b>312</b>. An output facet <b>330</b> of the modulator section <b>306</b> transmits the modulated optical signal. An anti-reflection coating <b>332</b> is deposited on the output facet <b>330</b> of the modulator section <b>306</b> to prevent undesired reflection from entering the laser cavity.

0067A modulator bias and data multiplexing circuit <b>322</b> has an output <b>324</b> that is electrically coupled to a modulation input <b>326</b> of the modulator section <b>306</b>. The modulator section <b>306</b> is an intensity modulator that modulates a CW optical signal that is generated by the laser section <b>304</b> with the data generated by the modulator bias and data multiplexing circuit <b>322</b>. The modulated optical signal is transmitted though the output facet <b>330</b> of the modulator section <b>306</b> and the anti-reflection coating <b>332</b>.

0068Many different types of modulator sections <b>306</b> can be used with an optical transmitter <b>300</b>. For example, the modulator section <b>306</b> can be a Franz-Keldysh-type electro-absorption modulator section. Such a modulator section <b>306</b> includes a section of waveguide with an active region of bulk semiconductor heterostructure material having a slightly larger bandgap energy than the photon energy of the optical signal being modulated. When the modulator bias and data multiplexing circuit <b>322</b> applies a reverse bias field to the modulation input <b>326</b> of the modulator section <b>306</b>, the absorption edge is lowered, thus reducing the light emitted.

0069The modulator section <b>306</b> can also be a modulated amplifier-type modulator. Such a modulator includes a gain section that can be formed of the same material as the gain section <b>308</b> in the laser cavity. Modulated amplifier-type modulators can achieve relatively broad optical bandwidth. In addition, the modulator section <b>306</b> can be a guide/antiguide-type modulator. Guide/antiguide modulators use refractive index effects to achieve intensity modulation. However, unlike other devices that use refractive index effects, such as Mach-Zehnder type modulators, these modulators do not generate large amounts of phase and sideband information in the transmission spectrum because they do not use interference effects.

0070In one embodiment of the invention, the optical transmitter <b>300</b> is specifically designed and fabricated to have at least one parameter that causes the optical transmitter <b>300</b> to generate a transmission spectrum with suppressed phase and sideband information. There are numerous physical parameters of the laser section <b>304</b> and the modulator section <b>306</b> that can be adjusted to change the amplitude and phase characteristics of the modulated optical signal in order to suppress phase and sideband information from the transmission spectrum.

0071For example, parameters of the laser section <b>304</b>, such as the wavelength, the optical mode structure, and the parameters of the output facet coating can be adjusted during design and fabrication to suppress phase and/or sideband information from the transmission spectrum. In addition, parameters specific to DBR and DFB laser devices, such as the grating parameters and the properties of the waveguides in the gain section <b>308</b>, the phase section <b>310</b>, and the grating section <b>312</b>, as well as the coupling parameters between these sections, can be adjusted during design and fabrication to suppress phase and/or sideband information from the transmission spectrum.

0072Also, parameters of the modulator section <b>306</b>, such as the extinction ratio, the polarization properties, the 3-dB bandwidth, the modulator chirp, the optical mode structure, the input third-order intercept (IIP<b>3</b>), the spurious free dynamic range (SFDR), the lateral index guide and antiguide profiles (for guide/antiguide-type modulators), and the output facet coating properties can be adjusted during design and fabrication to suppress phase and/or sideband information from the transmission spectrum.

0073In one embodiment of the invention, the operating conditions of the optical transmitter <b>300</b> are chosen so as to suppress phase and/or sideband information in the transmission spectrum. For example, the current generated by the laser bias circuit <b>316</b> and the resulting optical power received by the modulator section <b>306</b>, the bias voltage that is generated by the bias and data multiplexing circuit <b>322</b> and received by the modulator section <b>306</b>, and the operating temperature of the laser section <b>304</b> and the modulator section <b>306</b> can be adjusted during operation to suppress phase and/or sideband information from the transmission spectrum.

0074The optical transmitters described herein that generate optical signals with improved or optimal spectral and phase characteristics for transmission through an optical fiber link can be used for transmitting 10 G Ethernet data in multi-mode optical fiber transmission systems greater than 300 meters long. Error free transmission of optical signals having a 1310 nm wavelength over 300 meters of multi-mode optical fiber using such optical transmitters has been demonstrated.

EQUIVALENTS

0075While the invention has been particularly shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.